1. Field of the Invention
The present invention relates to a transmitter of a radio communication system and, specifically, to a device for controlling distortion characteristic of a predistorter which compensates the distortion of a transmitting signal by using ready-only memory (ROM) data, and to a method therefor.
2. Description of the Related Art
A non-linear characteristic generated in a high output transmitter of a radio transmitting system brings about the distortion of a transmitting signal, thereby causing the deterioration of the transmitting characteristic. For the sake of compensating the above deterioration, a non-linear distortion characteristic in opposition to the non-linear distortion characteristic caused in a high output amplifier of the above transmitter is beforehand generated in an input terminal of the above transmitter. The non-linear distortion characteristic generated in the predistorter reciprocally cancels the distortion characteristic caused in the high output amplifier disposed at a rear terminal of the predistorter. The output level of the high output amplifier for use in the radio transmitting system is adjusted by the input level of a counterpart's receiver, that is, the transmitting system has an automatic power control function. At this time, the distortion characteristic of the high output amplifier is varied according to the variation of the output level and, accordingly, the distortion characteristic of the predistorter is varied.
FIG. 1 is a detailed block diagram showing the construction of a general predistorter 1, and FIG. 2 is a diagram showing a transmitter for outputting a linear radio signal by using the predistorter of FIG. 1. In this instance, the predistorter 1 varies its distortion characteristic depending on the output level thereof. Here, it is a distortion control voltage Vc that varies the distortion characteristic of the predistorter 1.
An explanation on the operation of the predistorter 1 will be given briefly with reference to FIG. 1. The predistorter 1 receives a modulation signal a as an input signal thereof. Then, the received modulation signal a is amplified in a first amplifier 2 to have enough distortion in a distortion generator 12. Then, a power divider 4 divides the power in a delay compensator 6 and the distortion generator 12. The distortion generator 12 has its third distortion characteristic and then performs the distortion. Thereafter, an output signal of the distortion generator 12 is applied to a distortion adjuster 14 to adjust the size and the phase of the distortion, so that the adjusted size and phase can be combined with a delay compensated signal in the delay compensator 6 through a power coupler 8. At this moment, the distortion signal regarding the delay compensated signal has a phase difference of 180.degree. relative to the distortion characteristic of an output amplifier to be compensated. After the output of the power coupler 8 is amplified in a second amplifier 10 as much as that of the first amplifier 2, the amplified output is outputted as a distorted modulation signal B. The frequency spectrum of both the modulation signal A as the input signal of the power coupler 8 and the distorted modulation signal B as the output signal thereof is illustrated in FIG. 1.
The predistortion process performed in the predistorter 1 of FIG. 1 is naturally achieved in an intermediate frequency band.
With reference to FIG. 2, the distorted signal B outputted from the predistorter 1 of FIG. 1, for example, an intermediate frequency band, is inputted to an up-converter 20, the output C of which is provided to an output amplifier 22 of the radio frequency band, and is canceled with the non-linear distortion characteristic of the output amplifier 22, so that a linear radio signal can be outputted to a directional coupler 24.
At this moment, the distortion control voltage Vc is provided to the predistorter 1 in FIG. 2 to thereby vary the distortion characteristic depending on the output level of the predistorter 1. The processes of generating the distortion control voltage Vc are as follows. The directional coupler 24 divides the output of the output amplifier 22 and applies the divided output to the local down-converter 30. The radio signal outputted from the directional coupler 24 is converted into the intermediate frequency bandpass by the local down-converter 30 and the result is outputted through a bandpass filter 32 as an inter-modulation signal (hereinafter, referred to as IMD). The bandpass filter 32 performs bandpass filtering to minimize the power of the IMD. The IMD signal as the output of the bandpass filter 32 has its power level detected in the power level detector 34. A controller 36 provides the distortion control voltage Vc to the predistorter 1 in response to the detected power.
In the meantime, as depicted in FIG. 2, a local oscillator 28 provides a local oscillation signal and the power divider 26 divides the local oscillation signal of the local oscillator 28 to provide a divide signal to the up-converter 20 and the local down-converter 30.
However, it is a disadvantage that the transmitter as illustrated in FIG. 2, should be indispensably constructed with an additional circuit such as the local down-converter 30, the bandpass filter 32, the power level detector 34 and the controller 36 for the sake of providing to predistorter the distortion control voltage Vc for changing the distortion characteristic in accordance with the output level. Also, it is impossible to softly meeting the distortion characteristics of the predistorter 1 depending on the distortion characteristic of the output amplifier 22.